The Acute Respiratory Distress Syndrome (ARDS) is a common and frequently fatal condition, the hallmark of which is disruption of the alveolar-capillary barrier with the formation of non- cardiogenic, protein-rich pulmonary edema (1). ARDS is defined by the acute onset of bilateral opacities on chest radiograph, significant hypoxemia (PaO2/FiO2 <300), and exclusion of cardiogenic edema as the primary cause of hypoxemia (10). Unfortunately, despite decades of research, clinical trials have failed to identify any effective targeted therapies for ARDS, and treatment is currently limited to supportive care. In part, this failure to identify useful pharmacotherapies may be a result of the clinical and biological heterogeneity within ARDS. Our research group recently identified two subphenotypes within ARDS, with different natural histories, clinical characteristics, and biomarker profiles. Importantly, these groups responded differently to high versus low positive end-expiratory (PEEP) ventilation in a randomized, controlled trial (8). The results of this study indicate that understanding the subphenotypes of ARDS may allow more targeted management of patients with ARDS based on both biological and clinical characteristics. What remains unknown is if these subphenotypes will be identifiable outside of randomized controlled trials and if these phenotypes will have a differential response to other therapies. The overall objective of this proposal is to validate previously identified ARDS subphenotypes and to better define their biology, using data from both a large clinical trial as well as from a more inclusive observational cohort at UCSF. In Aim 1, we will test for the presence of distinct ARDS subphenotypes, including evaluation of differential response to therapy, in patients previously enrolled in the Fluid and Catheter Treatment Trial (FACTT). We hypothesize that using three variables previously shown to accurately discriminate subphenotype (interleukin-6, soluble tumor necrosis factor receptor-1, and vasopressor use), there will be two distinct subphenotypes of ARDS. Furthermore, these two subphenotypes will respond differently based on the randomized controlled trial of fluid management strategies. In Aim 2, we will test for the presence of distinct ARDS subphenotypes in a diverse prospective observational cohort of critically ill patients. We hypothesize that, using a similar approach as in Aim 1, there will be to distinct subphenotypes of ARDS in a more inclusive patient population at UCSF (the Early Assessment of Renal and Lung Injury [EARLI] cohort and that the hyperinflammatory subphenotype will be associated with higher mortality. In Aim 3, we will determine the degree of endothelial injury and lung epithelial injury within each ARDS subphenotype. We hypothesize that, in patients enrolled in the FACTT and EARLI studies, patients in Phenotype 2 will have a higher degree of endothelial injury, as measured by serum levels of Angiopoietin-2, while epithelial cell injury, as approximated by serum levels of receptor for advanced glycation endproducts (RAGE), will be similar between the two phenotypes.